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(NBC News) The FBI is examining thousands of photos from before and after Monday's deadly bombings at the Boston Marathon to determine whether nylon bags might have been placed in trash bags to appear less conspicuous.

Investigators believe the explosives used Monday were pressure cookers placed in nylon bags. The blasts killed three people and injured at least 176 others.

Among the photos under review are the two above, which were given to NBC station WHDH of Boston by a witness.

There was no way to know whether the bag in those photos is in any way relevant to the investigation, but the station provided them to the FBI for review, it said Tuesday.

The first picture shows a bag next to a mailbox along a barricade on the marathon route. The second — which the station said it had blurred because of its graphic nature — appears to show no sign of the bag.

The person who took the pictures told WHDH that as long as an hour may have passed between the times the two photos were taken.

Boston Police Commissioner Ed Davis said investigators were looking at thousands of photos and wanted as many as the public could send in to boston@ic.fbi.gov.

Jim Cavanaugh, a former special agent for the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives, said the experts know what to look for.

"They're able to recognize the minute pieces of an article that a civilian wouldn't even know ... but to a bomb investigator, like ATF or FBI, will say, 'That's from a clock; that's from a battery,'" Cavanaugh told NBC News